Method and apparatus for producing nonwoven fibrous fabric at high rate of speed

ABSTRACT

In the production of a nonwoven fabric of thermally bonded fibers, a heavy web of fibers is continuously fed to a toothed cylinder at a slow speed to form a layer of fibers, and a portion of this layer is removed and formed into a lightweight uniform web at a faster speed. The second web is conveyed without draw to a calender having a bonding nip, and the fibers of the web are rearranged by compression and heating and are supported on a hot surface of one of the calender rolls prior to entering the nip to additionally improve uniformity.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method and apparatus for rapidformation of a highly uniform nonwoven web of staple fibers and isparticularly suitable for the formation of the low bases weight webs ofthermoplastic fibers at a high rate of speed.

[0002] Nonwoven fabrics are produced by a variety of methods, and ingeneral, such methods involve the continuous laydown of fibers orfilaments in the form of an unconsolidated flat web on a conveyor,followed by consolidation of the web, such as by bonding or locking thefibers together to form the web into a cohesive fabric.

[0003] The carding of staple fibers into an unconsolidated web followedby point bonding with a hot calender is one well known method ofproducing a nonwoven fabric. In such a process the fibers, which arereceived in bales, are first opened with standard textile openingequipment. The opened fibers are then fed to single or multiple cardswhich are installed in line, each forming a thin web. The webs are thenlayered together, then usually spread to increase web width, and fed toa hot calender for thermal bonding. The customary calender consists oftwo heated rolls, one being a smooth steel anvil roll, the other being aroll with an embossed pattern. The high points of the pattern are thearea where the fibers are bonded together through partial melting. Suchsystems can produce webs which are reasonably uniform at a given speedand basis weight. Typically, a reduction in unit weight or an increasein speed results in a noticeable degradation in the uniformity of thefiber distribution. More precisely, at lower basis weights the webdevelops a more blotchy appearance due to areas of higher and lowerconcentrations of fibers. In the worst case, holes will form where theconcentration of fiber is low. The degradation in web uniformity for thetraditional system is also linked to the need of additional draw on theunbonded web to eliminate the bulging of the web which would otherwiseoccur at various points in the process. The amount of draw used tocontrol the web during transport to the calender is inverselyproportional to the cohesion of the unbonded web. A low cohesion webwill require a higher draw. The spreading section and the calender nippoint are prime areas where the bulging occurs. This bulging, if noteliminated, causes extremely poor web uniformity. A lighter web, whensubmitted to such increase in draw, develops even greater defectsbecause the extremely light areas are now deformed into holes in theweb.

[0004] The prior art has tried to minimize the requirement for draw byusing equipment transfer geometry and higher cohesion fiber to producenonwoven material at higher production speeds. Both modifications haveproduced only moderate improvements in speed or uniformity.

[0005] Other prior art has been the development of a machine whichreorganizes the carded unbonded web (with minimal or no increase inoutput speed) by reforming it on a vacuum collector such as described inU.S. Pat. No. 4,475,271. This process can produce a web with a moreuniform balance in tensile strength between the MD and CD direction but,it does not deliver the desired level of uniformity in fiberdistribution as judged by visual appearance.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, a slow moving thick orhigh basis weight web of fibers having a high degree of cohesion, isformed using conventional cards, or other mechanisms. This web may befirst spread in the cross machine direction.

[0007] The thick web is fed into a relatively fast moving toothedreforming roll, which carries a layer of excess recirculating fibersneeded to form the final web. A uniform portion of the layer of fibersis continuously removed from the reforming roll by a toothed web formingroll, and this web layer is transferred as a web to a conveyor by atransfer roll. The web is subsequently bonded.

[0008] In the preferred embodiment, the reformed web is fed from theconveyor around an air control transfer roll, which allows the web tochange direction without lifting or disruption, with the exit of the aircontrol roll being located closely adjacent the upper heated roll of therotating calender rolls.

[0009] The web is not fed directly into the nip between the calenderrolls. Rather, the web is transferred to the upper hot calender rollinto a secondary nip between the transfer roll and hot calender roll, inan area upstream of the nip. The unconsolidated web is then heated andcompressed in the secondary nip and is supported on the hot roll priorto entry into the calender nip to become thermally bonded.

[0010] As the web passes through the secondary nip, the web iscompressed, causing fibers to move relative to each other in a moreuniform arrangement. This effect is aided by contact of the web with theheated roll in which individual heated fibers may shrink, curl or relaxas they are being physically rearranged by compression. The rearrangedweb is partially wrapped and supported on the heated roll, which tendsto eliminate any bulging of the web due to passage through the calender.

[0011] Downstream of the reformer roll, all rolls and conveyor operateat substantially the same surface speed, and no substantial machinedirection draw is imparted to the reformed web due to transport orthermal bonding. Thus, very light weight or low cohesive webs may beprocessed at high speeds without any loss in uniformity, and, in fact,uniformity is increased in the final stages of processing.

[0012] In summary, the invention can be considered as having severalgeneral aspects. First, a web of staple fibers having a first basisweight and moving at a first speed is converted into a second, moreuniform web having a second, lower basis weight and moving at a second,higher, surface speed. This is accomplished by continuously metering alayer of fibers from the first web onto a rapidly rotating toothedcylinder and removing a uniform portion of said layer to form the secondweb moving at the second speed. The second web is subsequently bonded.

[0013] In a broad second aspect, a web of individual fibers, includingat least some thermally bondable fibers, is subjected to preconditioningimmediately prior to passage through a nip of a bonding calender. Thepreconditioning involves subjecting the web to heat and compressionwhich is sufficient to at least partially rearrange the fibers in a moreuniform array, but insufficient to thermally bond the fibers.

[0014] A third broad aspect comprises supporting a web of unbondedthermoplastic fibers on a heated surface immediately prior to entry intothe nip of a calender. The second and third aspects are preferablyaccomplished using a heated roll of the calender to heat, compress andsupport the web upstream of the bonding nip.

[0015] A fourth broad aspect is to support the web of individual fibersto be thermally bonded at a substantially constant surface speed betweenthe zone of formation and into and through the bonding zone in order tominimize any draw on the web after final web formation and to preventloss of uniformity due to draw.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a side schematic view of the overall apparatus forcarrying out the method of the present invention.

[0017]FIG. 2 is an enlarged portion of a first part of the apparatusshown in FIG. 1.

[0018]FIG. 3 is an enlarged portion of a second part of the apparatusshown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019]FIG. 1 shows the overall apparatus representative of a productionline capable of carrying out the various aspects of the presentinvention.

[0020] A relatively thick or high basis weight of a web 10 ofunconsolidated fibers is first prepared. The web 10 may be formed by useof one and preferably a series of a plurality of conventional cards 12which serve to separate clumps of fibers from a bale into individualfibers and to deposit the fibers via a take-off roll 14 onto a movingconveyor 16.

[0021] The web 10 comprises individual staple fibers which are capableof being bonded by conventional techniques. The initial part of thepresent method may be used to form uniform webs of fibers which aresubsequently consolidated by thermal or non-thermal means. Non-thermalmethods include techniques in which the surfaces of the fibers are notmelted or softened to achieve bonding, including techniques such aschemical or adhesive (liquid or solid) bonding and hydraulicentanglement. In such cases, polymer fibers having higher melting pointscan be employed, such as polyester and polyamide, as well as otherfibers such as polyolefin.

[0022] The web as initially formed may also be bonded by thermal methodssuch as the application of heat, pressure and heat, or by the use ofsonic techniques. Thermal bonding methods include, for example,through-air bonding using hot air, and passage of the web through thenip of a pair of heated calender rolls, one having an embossed surfacewith raised areas to define the bond sites. In such a case, fibers whichhave a relatively low melting point are used alone or in admixture withother fibers. Suitable thermoplastic fibers of this nature includepolyolefins, such as polyethyle and polypropylene, multi-componentfibers having an outer polyolefin surface, uand mixtures thereof. In thepreferred embodiment, the fibers or fiber mixtures are capable of beingbonded by passage through a conventional bonding calender.

[0023] The initially formed web 10, typically having a basis weight offrom about 30 to about 90 grams per square meter (gsm) may be conveyedfrom the conveyor 16 to a conventional spreader 18, which functions toincrease the width of the web 10 in the cross machine direction. Sincethe web is relatively thick and cohesive at this stage, the spreadingoperation does not cause excessive loss in gross uniformity. At thisstage, the web will be moving at a speed in the order of from about 50to about 80 meters per minute.

[0024] The above apparatus is conventional in nature and provides aninitial feed web for subsequent processing in accordance with thepresent invention. The use of any known process for opening andindividualizing fibers to form the initial web 10 is expected to sufficefor the purpose of the present invention.

[0025] In accordance with the present invention, the initial web 10 isfirst processed through a web reformer station 20 which results in ahighly uniform web 22 having a basis weight of from about 20 to about 70percent of the basis weight of the initial web, typically 10 to 30 gsm,and moving at a line speed of from about 150 to 500 percent greater thanthe line speed of the initial web, typically in the order of 150 to 250meters per minute. The web 22 is then conveyed to a final fiberrearrangement and bonding station 24, wherein the fibers are subjectedto additional mechanical and thermal rearrangement shortly prior tobonding.

[0026] The reformer station 20 is shown in FIG. 2, with the feed web 10and reformed web 22 being omitted between rolls for the sake of clarity.The initial web 10 exits a conveyor 26 and is deposited between a lowercurved support 28 and a toothed feed roll 30. The feed roll 30 metersfibers onto a toothed cylinder 32 operating at substantially a fastersurface speed and in the same direction (see arrows) than the feed roll.Semi-cylindrical covers 34 are preferably provided around the movingperiphery of cylinder 32 in a closely spaced relation to uniformly guidethe flow of air created by the cylinder and to prevent disturbance offibers residing thereon by outside influences. The fibers are not cardedby the cylinder 32, as this would reduce production speed.

[0027] A toothed forming roll 36 is provided, at a close distance fromthe cylinder 32 and rotates in an opposite rotary direction. Thecylinder 32 deposits a uniform layer of fibers resident as the outerlayer of fibers on the cylinder onto the forming roll 36. Thus, thecylinder 32 carries an amount of fibers in excess of that required toestablish the reformed or second web 22. As the cylinder 32 rotates pastthe feed roll 30, areas lacking a sufficient population of fibers toform a uniform layer will tend to pick up more fibers from the feed.Thus, the feed roll, cylinder and forming roll work in dynamicconjunction to provide a highly uniform web of unbonded fibers at a highrate of speed. The surface speed of the cylinder 32 is substantiallygreater than the surface speed of the forming roll 36, preferably in theorder of from about 3.5 to about 10 times faster.

[0028] A toothed take-off roll 38, located at a close distance from theforming roll 36 and rotating in an opposite direction, removes theentire reformed web 22 from the forming roll and deposits the same on amoving conveyor 40, which is preferably upwardly inclined relative tohorizontal machine direction travel.

[0029] The reformed web of individual fibers 22, which is now in ahighly uniform and fast moving state, may be consolidated or bonded byany suitable thermal or non-thermal technique as described hereinabove.Preferably, however, the web 22 comprises heat bondable fibers and issubjected to additional conditioning, followed by bonding by passagethrough a conventional heated calender having one or two pattern rolls.

[0030] In the preferred embodiment, the reformed web 22 is subjected tofinal processing and bonding at the station 24 as shown in FIG. 3. Theconveyor belt 40 is preferably of mesh construction allowing air flowtherethrough of at least 300 CFM per square foot. An air flow transferroll 42 supports the exit return loop of the conveyor belt 40. A pair ofspaced fixed radial air seals 44 and 46 are provided across the width ofthe roll 42. The first seal 44 intersects the belt 40 and the supportedweb 22 at approximately the 12 o'clock position on roll 42, as shown.

[0031] A calender apparatus is provided closely adjacent the airtransfer roll 42 and comprises an upper smooth heated roll 48 and alower embossed or patterned roll 50, rotating in opposite directions asindicated by the arrows as shown. In the alternative, the upper roll 48may have an embossed or patterned surface, and the lower roll 50 may bepatterned or smooth. The upper roll 48 is in tangential relation withthe air transfer roll 42 and is slightly spaced therefrom, as will beexplained in greater detail. A first nip 52 is defined between thecalender rolls 48 and 50, where thermal/pressure bonding occurs, and asecond nip 54, upstream of the first nip, is defined between the airtransfer roll 42 and the upper calender roll 48. The second seal 46intersects the second nip 54.

[0032] Suitable means, such as an air pump 56, are connected to a plenumchamber 58 to cause a uniform flow of air to be drawn through the porousconveyor belt 40 and into and across the web 22 in the zone between thefixed seals 44 and 46. Since the web will typically be light in weightand highly porous, the purpose of this air flow is not to provide apositive pressure drop or seal for the transfer process. Rather, thepurpose is to control the boundary layer air which would normally moveaway from the roll as speed is increased. The negative air flow allowsthe web to be transferred without disturbance and also prevents thepossibility of turbulence and hence disruptive forces at the second nip54.

[0033] It has been found that the nip 54 established between the rolls42 and 48 should be in the order or 0.250 in. (0.635 cm)or less. As thereformed web 22 enters the nip 54, the web is compressed between the tworolls, and the fibers in the web are heated by the hot calender roll.The simultaneous heating and compression causes at least a partialrearrangement of the fibers due to mechanical and thermal influences,allowing the fibers to shrink and relax as well as to move relative toone another and in three dimensions into the most efficiently packed oruniform arrangement while the fibers remain unbonded.

[0034] The web 22 adheres to and is supported by the heated calenderroll through a quadrant of rotation 60 until the web passes through thefirst nip 52 where permanent point bonding between the fibers occurs. Inprior art arrangements the web passes through an unsupported area priorto the nip of the calender, and due to compressive forces at the nip, abulge in the web can form prior to the nip, with the only availablesolution being to increase the machine direction draw on the web byincreasing the speed of the calender rolls relative to the speed of theweb feed. In the present arrangement, the final rearrangement of thefibers and the support of the web on the roll 48 serve to eliminate anytendency to bulge.

[0035] Apparatus of the prior art requires a substantial amount of drawto enable processing. In the present apparatus, the draw between theforming roll 36 and the bonding nip 52, if any, is less than 5% and mostpreferably less than 3%. Thus, the surface speed of all componentsdownstream of the cylinder 34 is substantially the same. As a result,bonded webs of a low basis weight and uniformity can be formed at aspeed up to 30-40% greater than available on a conventional line. As aresult, it is possible to produce light weight nonwoven webs of veryhigh uniformity and at high production rates and low cost, in comparisonto prior art methods.

What is claimed is:
 1. Apparatus for making a uniform web of fibers,said apparatus comprising a rotating toothed roll, means for supplying afirst web of fibers to said toothed roll at a first basis weight andproviding a layer of fibers on said toothed roll, and means for removingfrom said toothed wall a portion of said layer of fibers and forming asecond web of fibers having a basis weight lower than said first basisweight.
 2. The apparatus of claim 1 wherein said first web moves at afirst speed and said second web moves at a second speed faster than saidfirst speed.
 3. The apparatus of claim 1 wherein said tooth roll rotatesat a surface speed substantially faster than said means for removing aportion of said layer of fibers.